1. Field of the Invention
The present invention relates to a glass substrate for manufacturing a thermal-assisted magnetic recording disk and a thermal-assisted magnetic recording disk.
2. Description of the Related Art
With an increase of amount of information to be handled in the recent information processing, the recording capacity of a magnetic recording disk such as the hard disk is increased with each passing year, with which the recording density of the magnetic recording disk is also increased year after year. For example, in recent years, even a magnetic recording disk having an ultra-high recording density of 100 gigabytes per inch has been developed.
As a method of realizing the high recording density, it is often employed to lower the flying height of the magnetic head from the main surface of the magnetic recording disk. However, if there is an undulation with a period such that the magnetic head cannot approach the main surface of the magnetic recording disk, the flying height cannot be set beyond the surface roughness. To cope with the problem, for example, a technology for lowering the flying height is disclosed in Japanese Patent Application Laid-open No. 2000-200414, by suppressing the surface roughness of a glass substrate that becomes a base material for the magnetic recording disk with the average roughness of equal to or smaller than 1 nanometer and the maximum roughness of equal to or smaller than 15 nanometers.
Another method of realizing the high recording density includes a method of using a thermal-assisted magnetic recording system. In this method, when performing a magnetic recording, a laser device installed at near field illuminates an area where information is to be recorded with a laser light to lower the coercivity by increasing the temperature of the area, so that the magnetization of the area by the magnetic head becomes easy. By employing this method, because the recording magnetic portion can be formed using magnetic material having a high stead-state coercivity, it is possible to increase the recording density by narrowing the width of each recording track of a magnetic recording disk while preventing a loss of magnetization due to thermal fluctuation. Hereinafter, the magnetic recording disk employing the thermal-assisted magnetic recording method is referred to as a thermal-assisted magnetic recording disk.
However, in the thermal-assisted magnetic recording method, the temperature-elevated area is spread beyond the area where the recording is performed due to the thermal diffusion. As a result, a partial magnetization occurs also in a track adjacent to the recording track, resulting in a possibility of leading a degradation or a loss of data in the adjacent track, which is called the cross write. To cope with this problem, for example, in Japanese Patent Application Laid-open No. 2007-134004, a technology is disclosed in which the cross write is prevented by keeping an area where the temperature is elevated by the laser illumination from being spread to an adjacent recording track by separating each recording track with a non-magnetization portion having a thermal conductivity equal to or lower than one hundredth of that of the recording track.
Nevertheless, the conventional thermal-assisted magnetic recording disk still has a problem of the cross write even when the non-magnetization portion is provided between the recording tracks.